Bottom Line:
Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry.From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave.The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

ABSTRACTTerahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

f3: Fast electron images detected by stacked imaging plates.The images are taken for a single shot at d = 12 to 114 mm. The distance from the laser-irradiated spot to the end of the wire is 64 mm. The imaging plates used for d = 12 to 64 mm have a pinhole of 1 mm in diameter to allow the wire to pass through. The value indicated in each images is the integrated count of the electron signal. The color scale is set independently for each image, according to the indicated contrast scale factor, to provide maximum contrast.

Mentions:
First, the fast electrons emitted from the laser-irradiated spot in a direction along the wire (negative x-direction) were detected by triple-layer stacked imaging plates (IPs) at distances d = 12 to 134 mm, as shown in Fig. 3. The imaging plates are highly sensitive to electrons in the energy range from 40 to 1000 keV, with a sensitivity peak at around 200 keV. The second and third layers of the stacked imaging plates detect electrons with energies higher than about 400 and 600 keV, respectively. The electron distributions beyond the end of the wire (d > 64 mm) are very similar to those reported previously23. The total charge of the electrons detected at the first layer IP is estimated to be 1.5–3 nC and 1–1.5 nC for d ≤ 64 mm and d > 64 mm, respectively. The integrated counts of electron signals in the second layer IP are more than one order of magnitude lower than in the first layer IP, so it is estimated that more than 90% of the electrons have an energy lower than 400 keV. Thus, it is clear that the majority of the fast electrons have velocities substantially slower than the speed of light.

f3: Fast electron images detected by stacked imaging plates.The images are taken for a single shot at d = 12 to 114 mm. The distance from the laser-irradiated spot to the end of the wire is 64 mm. The imaging plates used for d = 12 to 64 mm have a pinhole of 1 mm in diameter to allow the wire to pass through. The value indicated in each images is the integrated count of the electron signal. The color scale is set independently for each image, according to the indicated contrast scale factor, to provide maximum contrast.

Mentions:
First, the fast electrons emitted from the laser-irradiated spot in a direction along the wire (negative x-direction) were detected by triple-layer stacked imaging plates (IPs) at distances d = 12 to 134 mm, as shown in Fig. 3. The imaging plates are highly sensitive to electrons in the energy range from 40 to 1000 keV, with a sensitivity peak at around 200 keV. The second and third layers of the stacked imaging plates detect electrons with energies higher than about 400 and 600 keV, respectively. The electron distributions beyond the end of the wire (d > 64 mm) are very similar to those reported previously23. The total charge of the electrons detected at the first layer IP is estimated to be 1.5–3 nC and 1–1.5 nC for d ≤ 64 mm and d > 64 mm, respectively. The integrated counts of electron signals in the second layer IP are more than one order of magnitude lower than in the first layer IP, so it is estimated that more than 90% of the electrons have an energy lower than 400 keV. Thus, it is clear that the majority of the fast electrons have velocities substantially slower than the speed of light.

Bottom Line:
Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry.From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave.The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

ABSTRACTTerahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.